Defects in additive manufactured metals and their effect on fatigue performance: A state-of-the-art review

https://doi.org/10.1016/j.pmatsci.2020.100724Get rights and content

Abstract

Additive manufacturing (AM) is emerging as an alternative to conventional subtractive manufacturing methods with the goal to deliver unique and complex net or near-net shaped parts. AM components should operate under various loading conditions, from static to complex dynamic multiaxial loadings, therefor, fatigue performance is often a key consideration. Intrinsic AM defects such as Lack of Fusion (LOF) defects, porosities, and un-melted particles are important for fatigue as a local phenomenon which usually starts at stress concentrations. Defects can be minimized by process optimization and/or post-processing but may not be fully eliminated. Full-scale testing, which is typically very costly and often necessary to assess reliability for fatigue performance of safety critical components, could be reduced by robust analytical fatigue performance prediction techniques. This work reviews the literature on the influential microstructural attributes on fatigue performance of AM parts with a focus on generated defects. This includes AM defect characterization and statistical analysis methods, as well as effect of process parameters and post-processing on defects, and consequently fatigue performance. The review also includes defect-based, microstructure-sensitive, and multiscale models proposed in the literature for modeling the effect of defects on fatigue performance and provides an outlook for additional research needed.

Introduction

Additive manufacturing (AM) is a method through which parts are created by additive processes as opposed to the conventional subtractive processes. These technologies were first targeting rapid prototyping to assist the design process. By the emergence of more advanced technologies, the properties of the manufactured parts improved to meet the expectations of various industrial purposes. These processes can currently utilize metals, ceramics, bioengineered tissues, and various polymers as the feedstock material.

The technology of AM, which is directly informed by 3D model data, is being developed rapidly as a potential production method in several industries. This technology offers significant advantages such as delivering intricate and complex geometries and short lead times. Powder and wire are forms of feedstock commonly used in metal AM. The powder or wire is melted by a focused heat source generated commonly by a laser beam or electron beam and subsequently cooled to form a part. As the AM technologies are being industrialized, the quality of the manufactured parts is improving, creating broader applications for these parts. Technologies like laser additive manufacturing (LAM) are being introduced exponentially to fields such as biomedical research, aircraft, and space industries. These methods can produce metal components with desirable properties for a variety of applications.

AM process parameters, the formed microstructure and defects, and consequently the mechanical properties of the manufactured parts have significant implications on their structural integrity. Operation of the components made by AM for applications such as in biomedical and aerospace industries, should be reliable under a wide variety of complex dynamic loadings in different environmental conditions. These loading conditions are oftentimes multiaxial even in parts under uniaxial loading due to complex geometry or interaction of residual stresses and presence of AM defects.

AM parts have shown a finer microstructure compared to the conventionally made parts, which leads to a relatively good static strength. Also, since defects have a less significant effect under static loading as opposed to cyclic loading, parts generally meet the standards and specifications on tensile properties for industrial use. Metal AM fatigue performance, on the other hand, is significantly affected by presence of defects. Comprehensive investigations are required to study the characteristics of AM defects and operation of AM components containing defects under dynamic loading conditions. High cost full-scale testing in environments resembling the working conditions, might be necessary to achieve the required level of reliability. However, testing could be reduced by development of applied analytical performance prediction techniques based on the intrinsic defects of AM parts.

Metal AM parts contain a variety of defects such as Lack of Fusion (LOF) defects that have also been reported in welds, un-melted particles as reported in Powder Metallurgy, and gas porosities, as detected in castings. Cracks could also rise from accumulated residual stresses in metal AM parts during the manufacturing process and the resultant distortion [1]. Since fatigue cracks usually start at stress concentrations like pores and inclusions [2], these defects have a significant effect on the fatigue life of AM components [3], [4], [5], and are the key contributors to the inferior fatigue properties of metal AM parts compared to their wrought counterparts [6], [7]. These imperfections also promote localized corrosion attacks and consequently stimulate fatigue cracking [8]. Therefore, metal AM defects should be characterized, analyzed, categorized, and ranked based on their importance and the degree that they affect fatigue performance. Non-destructive characterization methods combined with predictive models could help to predict fatigue performance based on the microstructural features and defect content.

Processing and post-processing strategies influence the fatigue performance of metal AM parts through altering microstructure and defects, and consequently the material sensitivity to the existing defects. AM processes should be optimized to improve fatigue performance of the AM parts. The main attributes to target to improve fatigue performance are residual stresses, surface roughness, internal defects, and microstructure. Variation of the final microstructure and defect content throughout the component might be seen even in one large component with complex geometry, mainly due to temperature gradient variations and heat transfer parameters at different locations during the process. Therefore, post-processing might be still necessary for metal AM parts to remove tensile residual stresses, achieve a uniform microstructure and defects content, and the desired reliability of fatigue performance. The challenge is to improve the quality of net shaped metal AM parts and to reduce the amount of required post-processing procedures.

Metal AM processes such as EBM and SLM and their comparison for various applications have been the focus of several papers such as works by Oliveira et al. [9], Liu and Shin [10], Zhang et al. [11], and Sing et al. [12] in which the different processes and their general mechanical properties has been discussed. Liu and Shin had gathered key fatigue properties for Ti-6Al-4V such as fatigue limit and fatigue long crack growth threshold based on the process (EBM, SLM, DED, wrought, cast, forged), post processing and surface finish, specimen orientation, and the load ratio. Their comparisons help to better understand the relations between processing parameters, resultant microstructures and associated mechanical properties which would be useful for modeling the fatigue performance of AM metals. They also have emphasized the critical effect of defects in as-built AM Ti-6A-l4V components on mechanical performances. They concluded that α′ martensite microstructure of DED and SLM Ti-6Al-4V are responsible for the lower crack thresholds, but higher fatigue limits as compared to EBM, wrought, forged and heat treated Ti-6Al-4V and confirmed the positive effect of surface machining and heat treatments on the fatigue performance of AM fabricated Ti-6Al-4V as will be further discussed in this work.

In recent years several review papers such as the works by Yu et al.[13], Lewandowski et al. [14], and Yap et al. [15] had addressed the state of the art in metal AM processes which contain valuable information on various AM technologies, their advantages and limitations, and the application of the AM metals based on their different properties. However, this review focuses on the effect of intrinsic metal AM defects, both surface and internal, on fatigue performance as one of the most critical properties of the AM metal components. As mentioned earlier AM metal components specifically in aerospace and biomedical applications need to perform under complex dynamic loading conditions and intrinsic metal AM defects significantly affect fatigue performance of these AM components.

A great fraction of the available literature is on metal PBF constant amplitude uniaxial fatigue performance. Also, AM Ti-6Al-4V fatigue performance is the most frequently studied. Typical metal AM processes and the sources of defect formation are mentioned, defect characterization and statistical analysis methods are explained, and the effect of these defects on fatigue performance of AM parts is discussed. Methods used throughout literature for modeling and prediction of the role of defects on the fatigue performance of AM parts are also reviewed. Finally, an outlook and perspective for future research is provided.

Section snippets

Metal AM processes and classifications

AM technologies are generally categorized, based on the state of material used, the mechanism by which layers of material are binding, and the source of energy which melts or softens the material [16], [17]. Metal AM processes categorized based on the accumulation method and energy input are shown in Fig. 1 [18]. Based on this flowchart, two main categories of technology used in metal AM are Power Bed Fusion (PBF) and Directed Energy Deposition (DED). The main power sources used in PBF are

Effect of AM defects on fatigue performance

A common representative dimension for a defect is the area parameter proposed by Murakami and Endo [129], which has been used in many works to calculate the stress intensity factor, its threshold value, and the fatigue strength of parts containing defects, including AM parts [130], [131], [132], [133]. This area is an effective area defined as a smooth contour circumscribing the irregular defect shape found on the fracture surface for various defect types based on fracture mechanics concepts,

Modeling the effect of defects on fatigue performance and life predictions of AM metals

Designing and building metal AM parts with required fatigue performance necessitates developing an integrated approach to link the process, structure, property, and performance [18]. Currently, modeling of these aspects, as well as topology and process optimization are main areas of research for AM [173]. Defect-sensitive fatigue life modeling is of great importance in linking AM metals fatigue performance to their structure and, subsequently, performance at the component level [174].

Numerous

Summary and perspective for future research

AM technologies have the potential of transforming the current manufacturing methods in the near future. The ongoing research could greatly facilitate the optimization of material design and AM process, based on industrial standards and qualification. Meanwhile, the cyclic mechanical properties of metal AM parts need to be evaluated and optimized since fatigue is a dominant mode of failure due to the intrinsic defects, as well as the cyclic nature of the loads applied to such parts. This was

CRediT authorship contribution statement

Niloofar Sanaei: Data curation, Formal analysis, Investigation, Visualization, Writing - original draft, Writing - review & editing. Ali Fatemi: Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (227)

  • L.-E. Loh et al.

    Numerical investigation and an effective modelling on the Selective Laser Melting (SLM) process with aluminium alloy 6061

    Int J Heat Mass Transf

    (2015)
  • V. Cain et al.

    Crack propagation and fracture toughness of Ti6Al4V alloy produced by selective laser melting

    Addit Manuf

    (2015)
  • T.M. Mower et al.

    Mechanical behavior of additive manufactured, powder-bed laser-fused materials

    Mater Sci Eng, A

    (2016)
  • G. Kasperovich et al.

    Improvement of fatigue resistance and ductility of TiAl6V4 processed by selective laser melting

    J Mater Process Technol

    (2015)
  • H. Zhang et al.

    Experimental study of effect of post processing on fracture toughness and fatigue crack growth performance of selective laser melting Ti-6Al-4V

    Chin J Aeronaut

    (2019)
  • D.M. Jafarlou et al.

    Structural integrity of additively manufactured stainless steel with cold sprayed barrier coating under combined cyclic loading

    Addit Manuf

    (2020)
  • X. Cui et al.

    Microstructure and fatigue behavior of a laser additive manufactured 12CrNi2 low alloy steel

    Mater Sci Eng, A

    (2020)
  • S. Romano et al.

    High cycle fatigue behavior and life prediction for additively manufactured 17–4 PH stainless steel: Effect of sub-surface porosity and surface roughness

    Theor Appl Fract Mech

    (2020)
  • R. Molaei et al.

    Multiaxial fatigue of LB-PBF additive manufactured 17–4 PH stainless steel including the effects of surface roughness and HIP treatment and comparisons with the wrought alloy

    Int J Fatigue

    (2020)
  • A. Riemer et al.

    On the fatigue crack growth behavior in 316L stainless steel manufactured by selective laser melting

    Eng Fract Mech

    (2014)
  • L. Thijs et al.

    Fine-structured aluminium products with controllable texture by selective laser melting of pre-alloyed AlSi10Mg powder

    Acta Mater

    (2013)
  • K. Solberg et al.

    The effect of defects and notches in quasi-static and fatigue loading of Inconel 718 specimens produced by selective laser melting

    Int J Fatigue

    (2020)
  • D.B. Witkin et al.

    Influence of surface conditions and specimen orientation on high cycle fatigue properties of Inconel 718 prepared by laser powder bed fusion

    Int J Fatigue

    (2020)
  • C. Pei et al.

    A damage evolution model based on micro-structural characteristics for an additive manufactured superalloy under monotonic and cyclic loading conditions

    Int J Fatigue

    (2020)
  • M.E. Aydinöz et al.

    On the microstructural and mechanical properties of post-treated additively manufactured Inconel 718 superalloy under quasi-static and cyclic loading

    Mater Sci Eng, A

    (2016)
  • N. Sanaei et al.

    Analysis of the effect of internal defects on fatigue performance of additive manufactured metals

    Mater Sci Eng, A

    (2020)
  • G. Kasperovich et al.

    Correlation between porosity and processing parameters in TiAl6V4 produced by selective laser melting

    Mater Des

    (2016)
  • X. Shui et al.

    Effects of post-processing on cyclic fatigue response of a titanium alloy additively manufactured by electron beam melting

    Mater Sci Eng, A

    (2017)
  • N. Sanaei et al.

    Defect characteristics and analysis of their variability in metal L-PBF additive manufacturing

    Mater Des

    (2019)
  • Y.R. Choi et al.

    Influence of deposition strategy on the microstructure and fatigue properties of laser metal deposited Ti-6Al-4V powder on Ti-6Al-4V substrate

    Int J Fatigue

    (2020)
  • N. Kang et al.

    Microstructure and strength analysis of eutectic Al-Si alloy in-situ manufactured using selective laser melting from elemental powder mixture

    J Alloy Compd

    (2017)
  • S.L. Sing et al.

    Selective laser melting of titanium alloy with 50 wt% tantalum: Effect of laser process parameters on part quality

    Int J Refract Metal Hard Mater

    (2018)
  • J.C. Wang et al.

    Selective laser melting of Ti–35Nb composite from elemental powder mixture: Microstructure, mechanical behavior and corrosion behavior

    Mater Sci Eng, A

    (2019)
  • A.G. Demir et al.

    Multi-material selective laser melting of Fe/Al-12Si components

    Manufacturing Letters.

    (2017)
  • S.L. Sing et al.

    Interfacial Characterization of SLM Parts in Multi-material Processing: Intermetallic Phase Formation between AlSi10Mg and C18400 Copper Alloy

    Mater Charact

    (2015)
  • J. Chen et al.

    Interfacial microstructure and mechanical properties of 316L /CuSn10 multi-material bimetallic structure fabricated by selective laser melting

    Mater Sci Eng, A

    (2019)
  • C.F. Tey et al.

    Additive manufacturing of multiple materials by selective laser melting: Ti-alloy to stainless steel via a Cu-alloy interlayer

    Addit Manuf

    (2020)
  • J.W. Pegues et al.

    Fatigue of additive manufactured Ti-6Al-4V, Part I: The effects of powder feedstock, manufacturing, and post-process conditions on the resulting microstructure and defects

    Int J Fatigue

    (2020)
  • Y. Cao et al.

    Comparative investigation of the fatigue limit of additive-manufactured and rolled 316 steel based on self-heating approach

    Eng Fract Mech

    (2020)
  • M. Dallago et al.

    On the effect of geometrical imperfections and defects on the fatigue strength of cellular lattice structures additively manufactured via Selective Laser Melting

    Int J Fatigue

    (2019)
  • G. Strano et al.

    Surface roughness analysis, modelling and prediction in selective laser melting

    J Mater Process Technol

    (2013)
  • N. Sanaei et al.

    Analysis of the effect of surface roughness on fatigue performance of additive manufactured metals

    Theorit Appl Fract Mech

    (2020)
  • J. Gockel et al.

    The influence of additive manufacturing processing parameters on surface roughness and fatigue life

    Int J Fatigue

    (2019)
  • S. Bagehorn et al.

    Application of mechanical surface finishing processes for roughness reduction and fatigue improvement of additively manufactured Ti-6Al-4V parts

    Int J Fatigue

    (2017)
  • M. Benedetti et al.

    Low- and high-cycle fatigue resistance of Ti-6Al-4V ELI additively manufactured via selective laser melting: Mean stress and defect sensitivity

    Int J Fatigue

    (2018)
  • H. Zhang et al.

    The effects of ultrasonic nanocrystal surface modification on the fatigue performance of 3D-printed Ti64

    Int J Fatigue

    (2017)
  • D. Greitemeier et al.

    Fatigue performance of additive manufactured TiAl6V4 using electron and laser beam melting

    Int J Fatigue

    (2017)
  • M. Seifi et al.

    Effects of HIP on microstructural heterogeneity, defect distribution and mechanical properties of additively manufactured EBM Ti-48Al-2Cr-2Nb

    J Alloy Compd

    (2017)
  • A. du Plessis et al.

    Hot isostatic pressing in metal additive manufacturing: X-ray tomography reveals details of pore closure

    Addit Manuf

    (2020)
  • S. Tammas-Williams et al.

    Porosity regrowth during heat treatment of hot isostatically pressed additively manufactured titanium components

    Scr Mater

    (2016)
  • Cited by (468)

    View all citing articles on Scopus
    View full text